Hydrologic data were collected near Summit Lake Reservoir for 2 years beginning October 1, 1988, and ending September 30, 1990. The data-collection network consisted of 1 reservoir-stage gage, 2 precipitation gages, 1 evaporation pan, 2 streamflow gages, and 13 observation wells. Stage-area and stage-storage relations for the reservoir were used in combination with the hydrologic data to determine the reservoir's annual hydrologic budgets for water years 1989 and 1990.

Components of the hydrologic budget are considered either as inflow or outflow. Differences between inflow and outflow result in a change in reservoir storage. Components of inflow are direct precipitation, surface-water flow, and ground-water flow. Components of outflow are evaporation and ground-water flow. There was no surface-water outflow during the study.

The hydrologic budget was calculated by use of daily, monthly, and yearly values of precipitation and surface-water flow, monthly values of ground-water flow, and yearly values of evaporation and reservoir storage. Comparison of results from different time intervals indicates there is little difference between annual volumes for components calculated from daily and monthly values. Annual volumes calculated from yearly values differ by less than 5 percent from those calculated from daily and monthly values.

The total inflow to Summit Lake Reservoir was nearly identical for water years 1989 and 1990-about 10,360 acre-feet per year. Surface water was about 72 percent of the total inflow in water year 1989 and about 70 percent in water year 1990. Direct precipitation was about 18 percent of the total inflow in water year 1989 and about 23 percent in water year 1990. Ground-water inflow was about 10 percent of the total inflow in water year 1989 and about 7 percent in water year 1990.

The total outflow from evaporation and ground-water flow was 5,700 acre-feet in water year 1989 and 7,710 acre-feet in water year 1990--about 19 percent evaporation and 81 percent ground-water outflow for both water years. Reservoir storage increased during both years; the change in storage was about 4,330 acre-feet in water year 1989 and about 2,890 acre-feet in water year 1990. Discrepancies between inflow, outflow, and reservoir storage reflect errors in the budget resulting from regionalization or interpretation of hydrologic data and from errors inherent in all hydrologic measurements.